Factors influencing the atmospheric depositional fluxes of stable Pb, 210Pb, and 7Be into Chesapeake Bay

Atmospheric depositional fluxes of Be-7 and Pb-210 (bulk) and stable Pb (wet) were measured simultaneously for one year (from September 1995-August 1996) at Stillpond, Maryland on the upper eastern shore of the Chesapeake Bay. The annual total (bulk) depositional fluxes of Pb-210 and Be-7 were 0.78 and 13 dpm cm(,)(-2) respectively, and agree well with other previously reported results at nearby locations such as Norfolk, VA and Lewes, DE. The wet depositional flux of stable Pb (58 ng cm(-2) yr(-1)) was also similar to that measured at other Chesapeake sites during 1990-1991 (55 ng cm(-2) yr(-1), for both Wye and Elms, Maryland). This suggests that a constant Pb flux has been reached since the mandatory use of unleaded gasoline was instituted. The concentrations of Be-7, Pb-210, and to a lesser extent stable Pb were diluted exponentially by precipitation, based on concentrations versus precipitation plots. Due to higher enrichment of Pb-210 in the lower troposphere, the dilution effect was largest on Pb-210 (i.e., controlled mainly by below-cloud scavenging processes), and thus its depositional flux increase is negligible as precipitation amount increases. A good correlation between the amount of precipitation and total depositional flux of Be-7 and stable Pb, which are more enriched in the upper troposphere, suggests that precipitation amount is important in controlling their fluxes (i.e., controlled by both below-cloud scavenging and in-cloud condensation processes). Based on Be-7 versus Pb-210 plots, it appears that Be-7, relative to Pb-210, is less efficiently scavenged by snowfall. Our results suggest that in addition to precipitation amounts, marine air-mass transport or snowfall may be important factors in controlling the seasonal variability of the fallout fluxes of trace metals in coastal areas.